Developers of information storage devices and methods continue to seek increased storage capacity. As part of this development, holographic memory systems have been suggested as alternatives to conventional memory devices. Holographic memory systems may be designed to record data as one bit of information (i.e., bit-wise data storage). See McLeod et al. “Micro-Holographic Multi-Layer Optical Disk Data Storage,” International Symposium on Optical Memory and Optical Data Storage (July 2005). Holographic memory systems may also be designed to record an array of data that may be a 1-dimensional linear array (i.e., a 1×N array, where N is the number linear data bits), or a 2-dimension array commonly referred to as a “page-wise” memory system. Page-wise data storage or memory systems may involve the storage and readout of an entire two-dimensional representation, e.g., a page of data. Typically, recording light passes through a two-dimensional array of low and high transparency areas representing data, and the system stores, in three dimensions, the pages of data holographically as patterns of varying refractive index imprinted into a storage medium. See Psaltis et al., “Holographic Memories,” Scientific American, November 1995, where holographic systems are discussed generally, including page-wise memory systems.
In a holographic data storage system, information is recorded by making changes to the physical (e.g., optical) and chemical characteristics of the holographic storage medium. These changes in the holographic medium take place in response to the local intensity of the recording light. That intensity is modulated by the interference between a data-bearing beam (the data beam) and a non-data-bearing beam (the reference beam). The pattern created by the interference of the data beam and the reference beam forms a hologram which may then be recorded in the holographic storage medium. If the data beam is encoded by passing the data beam through, for example, a spatial light modulator (SLM), the hologram(s) may be recorded in the holographic storage medium as an array of light and dark squares or pixels. The holographic storage medium or at least the recorded portion thereof with these arrays of light and dark pixels may be subsequently illuminated with a reference beam (sometimes referred to as a reconstruction beam) of the same or similar wavelength, phase, etc., so that the recorded data may be read.
One type of holographic storage medium used recently for such holographic data storage systems are photosensitive polymer films. Photosensitive polymer films are considered attractive recording media candidates for high density holographic data storage. These photosensitive polymer films have a relatively low cost, are easily processed and may be designed to have large index contrasts with high photosensitivity. These photosensitive polymer films may also be fabricated with the dynamic range, media thickness, optical quality and dimensional stability required for high density applications. See, e.g., L. Dhar et al, “Recording Media That Exhibit High Dynamic Range for Holographic Storage,” Optics Letters, 24, (1999): pp. 487 et. seq; Smothers et al., “Photopolymers for Holography,” SPIE OE/Laser Conference, (Los Angeles, Calif., 1990), pp. 1212-03.
The holographic storage media described in Smothers et al., supra contain a photoimageable system containing a liquid monomer material (the photoactive monomer) and a photoinitiator (which promotes the polymerization of the monomer upon exposure to light), where the photoimageable system is in an organic polymer host matrix that is substantially inert to the exposure light. During writing (recording) of data into the holographic medium, the monomer polymerizes in the exposed regions. Due to the lowering of the monomer concentration caused by the polymerization, photoactive monomers from the dark, unexposed regions of the material diffuses to the exposed regions. The polymerization and resulting diffusion create a refractive index change, thus forming the hologram representing the data. An important aspect to these systems is the mass transport from one region to another to create a large change in refractive index, which may provide a distinct advantage over photochromic systems. Unfortunately, the continued mass transport may produce negative effects after the writing process has been completed.
The characteristics and capabilities of the holographic storage medium may depend upon or be affected by a number of factors, and especially the nature, properties, composition, etc., of the holographic storage medium. For example, the optical and chemical characteristics of the holographic storage medium may affect how the holographic storage medium absorbs different wavelengths of light, the speed with which a particular wavelength of light is absorbed, how well or uniformly the medium records the holograms with respect to the particular wavelength of light, etc. In addition, the recording characteristics of the holographic storage medium may change as the various chemical components present in the holographic storage medium are used up or formed, as the holographic storage medium ages over time, etc. All of these factors may affect and may make less optimal the characteristics and capabilities of the holographic storage medium to record and/or read data.